The invention relates to an apparatus for manufacturing semi-finished products and molded articles of metallic material incorporating an extruder for producing a metal flow and appliances connected thereafter for molding the semi-finished products and the molded articles.
A device of this type for die-casting preforms is known from EP 0 080 787, wherein a metallic material having dendritic properties, a magnesium alloy for example, is converted into a thixotropic state in an extruder. In this state, the metallic material has a mud-like or pasty consistency and can be processed so as to form metallic molded articles in the molding appliances following the extruder.
From EP 0 080 787, it is known to use a die-casting unit connected after the extruder for the molding or shaping operations, or else to process this material directly in a conventional injection molding machine without previously processing the metallic material in an extruder. There however, processing in an injection molding machine is disregarded because of the superimposition of rotational and translational movements by the injection worm (reciprocating screw) and the increased problems of sealing arising thereby in comparison with the use of the extruder (merely rotational).
The process of converting the metallic material (e.g. the magnesium alloy) into a thixotropic mass in the extruder is effected, in the manner described in EP 0 080 787, by feeding the material in granular form into a pre-heated feed hopper, whereby the size of the granular particles is made such that they can be easily processed by the screw in the extruder. The heating of the granules is effected at a temperature which is close to or above the solidus temperature whereby the heating process may take place either prior to and/or in the extruder.
The metallic material is in any case subjected to further heating in the extruder by means of external heating devices that are effective via the screw cylinder, and also as a result of frictional heat (shear stress). Hereby, the heating process in the extruder is controlled in such a manner that the temperature of the metallic material will remain below its liquidus temperature.
Due to the maintenance of a temperature in the range between the solidus and the liquidus temperatures and also due to the shear stress, the effect achieved in the extruder is that the dendritic structures of the metallic material will be broken down and a solid-liquid metal alloy in a thixotropic state will emerge from the output of the extruder.
In this device for producing a solid-liquid thixotropic metal alloy which is known from EP 0 080 787, there is a feed channel in the form of a continuous helical channel between the flanks of the screw from the start of the screw up to the end thereof.
Basically, the underlying principle of the conveying process in an extruder is that the material being moved experiences friction against the cylinder walling of the extruder and glides over the so-called base of the screw. When processing metallic materials, the problem arising as a result of the high thermal conductivity is that there is a build up of a smelt film on the cylinder walling, said film being of very low viscosity and considerably reducing the friction between the material being moved and the cylinder walling thereby leading to a drastic reduction in the performance of the conveying process. Moreover, the mixing process also suffers to a considerable extent whereby a growing temperature gradient over the cross-section of the interior of the extrusion cylinder, which gradient increases from the exterior to the interior thereof, cannot be effectively dissipated.
As a consequence of these conditions, there arise inhomogeneities between the solid and liquid components and the stability of the conveying process becomes extremely unsatisfactory whilst the build up of pressure is highly erratic. Continuously altering process states thereby arise whereby the resultant non-reproducible quality of the components has to be accepted.
It is therefore desirable to improve the construction and functionality of an apparatus of the type mentioned in such a manner that reproducible component-qualities can always be produced.
According to one aspect of the invention, an extruder for producing a flow of metal and appliances connected thereafter for shaping the semi-finished products and the molded articles includes a screw system consisting of two or more meshing screws.
In the case of the apparatus in accordance with the invention, the processing of the metallic material, for example, starting from the granular state up to the thixotropic solid-liquid material or the liquid material states thereof, is effected in such a manner that, taken with reference to the axial length of the extruder, the processing steps will generally be consistent and the material will be continuously advanced. The negative consequences of fluctuations in temperature and the irregularities of viscosity inherent therein together with the proportional composition of the liquid material components are thereby reduced to a negligible amount.
It has been found that the previously described problem encountered in single screw extruders can be eliminated by means of an extruder including a screw system comprising two or more meshing screws, although the problems described above still have to be taken into account initially even with this type of extruder. Here however, there is a counteracting mechanism at work which, surprisingly, is of sufficient extent as to allow the material being conveyed to be transferred from one screw onto the adjacent meshing screw. It is evident thereby, that when processing metallic materials, an adequately large mixing and transportation effect is achieved which ensures continuous advancement of the metallic material being conveyed in addition to the dissipation of the temperature gradient. Stable feeding of fresh material into the inlet zone of the extruder has also been observed in addition to the increase in mixing performance.
This has a particularly positive effect when processing materials in granular or chip-like form since the bulk density thereof of typically approximately 0.5 to 0.8 g/cm3 has to be doubled or brought up to the still higher densities of approximately 1.7 g/cm3 or more of the solid-liquid stream of material. This is difficult if not impossible in the case of the reduced conveyor performance of an extruder.
Heating strips or heating devices functioning inductively are used conventionally for the purposes of introducing heat when processing metallic smelts.
However, inductive heating devices are very expensive. Classical heating strips are mounted around the periphery of the extruder cylinder engine and tend to become heavily oxidized at the high temperatures prevailing when processing metallic smelts, this leading to scaling of the cylinder surface and hence reducing thermal transfer between the heating body and the cylinder. Moreover, precautions have to be taken when using heating strips so as to retain them in continuous contact with the surface of the cylinder in order to achieve adequate thermal transfer.
Another disadvantage associated with heating strips is the large spacing between the heating strips mounted externally on the extruder cylinder and the smelt present in the interior of the cylinder in the face of the necessarily high heat flow densities and temperature gradients of up to 200xc2x0 C. and more which occur in operation.
If the ratios involved even in the case of a single screw extruder are not particularly favorable, then the heat introduction ratio is still less favorable in the case of two and more screw extruders since the spacing between the outer cylinder surface and the inner walls thereof is inevitably increased here due to the geometrical considerations.
In accordance with the invention, so-called heating cartridges, which comprise resistance heating elements arranged in a usually cylindrical housing, are of assistance here.
The heating cartridges can be arranged in transverse bores in the cover of the extruder cylinder very close to the inner walling of the cylinder, for example, above and below the double cylinder chamber in the two screw extruder. The transverse bores themselves may be hermetically sealed using an airtight and heat resistant material so that they will be protected from scaling.
Substantially greater heat flow densities can be obtained due to the very small spacing between the heating cartridges and the inner walling of the extruder cylinder. Moreover, the outer surface of the extruder cylinder can be insulated to a still greater extent against loss of heat by the use of the heating cartridges arranged in the cylinder walls.
The extruder together with the heating cartridges mounted therein can be produced in such a manner that the tie rods thereof are arranged outside the insulating means and thus located in a considerably cooler region. Substantially more economical materials can thereby be used therefor.
The driving arrangement for the extruder screws as well as the driving arrangement for the die-casting pistons is often implemented by means of hydraulic systems.
However, there is a certain safety risk in regard to the combustibility of the hydraulic fluids due to the high temperatures occurring when processing metal smelts.
Accordingly, electrical drives are preferably used for the screws, but so too, electrical drives could also be used for driving the die-casting pistons rather than a hydraulic system.
Granular materials having dissimilar shaped grains can now be processed by means of the method in accordance with the invention and thus, in toto, there is a considerably broader spectrum of starting materials available, whereby one can resort to more economical starting materials.
Furthermore, due to the continuous conveying process, the band width of the period in which the materials being conveyed will remain in the extruder is reduced, this being shown by a uniform grain size in the globulites in the structure of the finished preforms or semi-finished products.
The previously described effects are immediately apparent if the screws rotate in the same sense. The positive effect is strengthened by using closely engaging screws.
Alternatively, screws rotating in the opposite sense could also be used, whereby an enforced advancement process would then be implemented here.
The extruder may be followed by one or more die-casting moulds which are adapted to be loaded with metallic material on a continuous or discontinuous basis via multi-way switches and heated channels.
A reduction of the production cycle can thereby be implemented, or larger components, especially thin-walled large surface area components can be manufactured, whereby a plurality of die-casting units can be connected to a molding cavity.
Due to the controllable processing states that are always running uniformly in the extruder, it is particularly suited for side feeding of differing metallic and non-metallic materials, especially of reinforcing components such as fibers for example.
Side feeding may be effected by means of a volumetric or gravimetric metering system.
Side feeding of the differing materials is effected at those functional and temperature zones which are appropriate for the respective materials. Pure metals e.g. Li, Mg, Ca, Al, Si, Zn, Mn, rare earth metals and the like can thus be compounded to form metal alloys, and, by the same token, pre-existing alloys such as e.g. AlZn can be supplied to the extruder in accordance with the invention. Moreover, non-metallic materials such as e.g. reinforcing materials, fillers, seeding agents, catalysts and the like can also be worked into the solid-liquid or liquid metal flow in this manner, whereby the extruder in accordance with the invention fulfils the function of a machine for manufacturing alloys or compound materials.
In addition, pre-prepared and especially liquid materials can also be supplied via side feeding by the previously proposed aggregates such as the extruders for example.
Basically, components of constant quality are thereby producible, whereby they may consist of pure metal, metal alloys or of non-metallic materials mixed homogeneously with the metal or the metal alloys.
The appliances connected to the output of the extruder for molding the semi-finished products and preforms may be selected from a large range. To mention just some of the most important:
Die-casting aggregates.
Continuous molding and extrusion aggregates.
In the case of die-casting aggregates, one should mention those die-casting aggregates that are equipped with a separate piston/cylinder unit such as are known from EP 0 080 787 for example.
Hereby, one should differentiate between the various types:
piston/cylinder aggregates which are filled at the front face of the piston, whereby the piston is in the withdrawn position at the beginning of the filling operation in the case of one variant and the filling operation takes place either directly in front of the piston or at a position displaced therefrom in the direction towards the cylinder opening; in an alternative variant, filling takes place at the cylinder opening and the piston is driven back or forced back during the filling operation, and in a further variant, the filling operation takes place in the cylinder chamber in front of the piston in the cylinder outlet channel and the piston is moved from the forward dead position by the inflowing metallic material into the withdrawn position.
In another embodiment, a so-called differential piston subdivides the cylinder chamber of the die-casting cylinder into a feed chamber connected via a heated channel to the extruder and an injection chamber connected to the molding cavity. A fluidic connection is created between the feed chamber and the injection chamber, said fluidic connection incorporating a return-flow blocking device or a non-return valve which counteracts any return flow of metallic material from the injection chamber into the feed chamber.
The differential piston has a greater area of piston surface at the injection chamber side thereof and a smaller, usually annular piston surface at the feed chamber side thereof.
The thixotropic metallic material that it is fed by the extruder into the feed chamber at a pressure of e.g. less than 120 bar is brought up to the injection pressure of e.g. 500 bar or more, especially 1000-2000 bar, by means of the differential piston, whereby losses due to leakage play no part since the leaked quantities entering the feed chamber from the injection chamber will be fed back into the injection chamber during the next injection phase.
Another advantage of the differential piston is that the proportionately low pressure in the material fed into the die-casting cylinder automatically returns the differential piston due to the pressure difference set up between the larger piston surface and the smaller annular piston surface, whereby the insertion of multi-way valves between the extruder and the die-casting cylinder is thereby redundant. If necessary, this process can be assisted hydraulically. Finally, there also arises the advantage that the flow of metallic material produced by the extruder is always advanced in just one direction towards the injection process in the molding cavity, this being particularly appropriate when processing materials into which long reinforcing fibers (e.g. carbon fibers) are to be worked and said fibers enter the extruder by side feeding.
Furthermore, the invention relates to a method of die-casting, continuous casting or extrusion molding metallic materials using an extruder followed by units for shaping semi-finished products and preforms, especially of the type described above. The use of an extruder incorporating a screw system comprising two or more meshing screws permits the metallic material to be conveyed in the direction of extrusion in a controlled manner. This also applies especially for materials in the solid-liquid thixotropic state as well as for materials in the liquid state.
Discontinuities occurring when processing the metallic material are avoided by virtue of the controlled advancement process or the enforced advancement process, this making a considerable contribution to improved consistency in the component quality of the metallic components produced in the die-casting process.
The processing of the metallic material in accordance with the invention and the controlled or enforced conveyance thereof in the extruder now permits, in a particularly simple and defined manner, the side feeding of further components, for example alloying components when manufacturing alloys, reinforcing components when manufacturing metallic compound materials, or other additional materials for modifying the metallic materials.
In particular, the controlled or enforced conveyance in the extruder ensures greater homogeneity of the metallic material produced.
In a series of cases, it is also useful to work at or above the liquidus temperature, especially in a range of approximately 5xc2x0 C. to 10xc2x0 C. above liquidus.
Working above liquidus is to be recommended in some cases of application since the mixing process is further assisted here and this improves especially the wetting of the added fibers.
An embodiment of the apparatus in accordance with the invention will now be described with reference to the drawing.